Many risk factors for non-insulin-dependent diabetes mellitus (NIDDM), such as obesity, physical inactivity, and high-fat diet, can potentially be modified. Furthermore, some of the metabolic abnormalities, such as insulin resistance and impaired glucose tolerance, that predict diabetes can be improved by behavior modification and drug treatment. Thus, at least to some extent, NIDDM may be preventable. Several small clinical trials have addressed the hypothesis that NIDDM can be prevented by dietary modification, physical activity, or drug treatment. Some studies suggest a preventive effect, but the conclusions are limited by considerations of sample size, randomization, or intensity of the interventions. Consequently, the hypothesis that NIDDM is preventable requires further testing.